The present invention relates to an improved molten metal filtration system having dam means for collecting a volume of molten metal sufficient to enable a reduction in the heat lost in the molten metal as it flows through the filtration system towards a casting nozzle and to thereby substantially prevent freezing problems in the molten metal being cast. The invention also relates to a process for using the system.
In practice, molten metal generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified. These inclusions adversely affect the physical properties as well as the aesthetic appearance of the final cast product.
A variety of different techniques are known for removing impurities from molten metal. U.S. Pat. No. 3,006,473 to Gamber illustrates one such system in which molten metal flows through a porous carbon filter plate in a filtering chamber. The system includes an overflow-type metal outlet which is constructed so that the molten metal is maintained at least above the top of the exposed portion of the filter plate.
In some systems, filtration of the molten metal takes place in a chamber having gas inlet device(s) for admitting a non-reactive gas. U.S. Pat. No. 4,087,080 to Steinegger et al. illustrates one such system. In the Steinegger et al. system, the melt flows into a filter chamber containing a loose bed of granulate and means for introducing gas into the melt. The melt flows downward through the filter chamber into a riser chamber, then upwards through the riser chamber, and finally out of the riser chamber.
In yet another system, the molten metal flows through a series of successively arranged purification stages including: (1) a deslagging stage wherein relatively large particulate impurities are removed from the molten metal by filtration through a woven refractory filter; (2) a fluxing stage for removing entrapped and dissolved hydrogen from the molten metal; (3) an adsorption stage; and (4) a final filtration stage wherein finer particulate impurities are removed by a rigid, porous refractory filter medium. U.S. Pat. No. 4,007,923 to Chia illustrates such a system.
Porous ceramic foam materials are known to be particularly useful in filtering molten metal. U.S. Pat. Nos. 4,024,056 to Yarwood et al., 4,092,153 to Yarwood et al., 4,277,281 to Weber et al., and 4,640,497 to Heamon illustrate various filtration systems employing ceramic foam filter materials. Ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons included among which are their excellent filtration efficiency, low cost, ease of use and disposability.
Barriers have been used in intermediate pouring vessels such as tundishes to assist in the separation of non-metallic inclusions from the molten metal passing through the tundish. They also have been used to assist in the escape of entrained gasses from the molten metal. U.S. Pat. No. 4,619,443 to Mitchell illustrates one such dam barrier. The dam has passages in its upper surface for discharging gas into the melt. It is intended to improve the circulation of the molten metal in the tundish so as to substantially eliminate dead spaces.
As shown in the aforementioned Heamon patent, filtration systems have been employed in the tundish of a continuous caster. The filter(s) of the system separate the tundish into one or more compartments. The incoming metal passes through the filter(s) prior to being teemed through the tundish nozzle. During the filter priming process, a certain amount of heat in the metal is lost to the filtration system. The heat loss encountered during this phase can be particularly great. FIG. 1 is a graph depicting typical temperature losses over time for molten stainless steel alloy passing through a tundish having a center compartment for receiving the molten metal and two outlet compartments separated from the central compartment by a 38 mm thick, #10 zirconia-alumina filter. The heat losses were measured using a thermocouple type temperature probes. As can be seen from the Figure, temperature losses occur on both sides of the filter assemblies with the initial temperature losses for the molten metal being filtered quite significant.
It has been found that at normal operating metal temperatures, heat losses can be sustained which are sufficient enough in some cases to cause the filtered metal to freeze in the nozzle of a post-filtration compartment and halt the cast. In many instances, the overall heat loss as measured by a temperature probe would indicate there is sufficient heat in the system to cast; however, the metal in the nozzle has in reality been chilled enough to freeze and prevent the cast.
Accordingly, it is an object of the present invention to provide an improved filtration system which reduces the heat losses sustained during the filtering process.
It is a further object of the present invention to provide an improved filtration system as above having utility in a continuous casting system.
It is still a further object of the present invention to provide an improved filtration system as above having utility in the casting of ferrous metal and metal alloys.
These and other objects and advantages will become more apparent from the following description and drawings in which like reference numerals depict like elements.